The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
The “Next Generation” 737 (the “737NG”) was designed to fly higher, faster, carry heavier loads, and be more fuel efficient than its predecessor, the 737 Classic, which is manufactured by the Boeing Company. However, airlines flying the 737 Classic at the time of the planned upgrade wanted to limit changes to specific features of the 737 Classic to control costs associated with the upgrade. Specifically, the airlines wanted the 737NG to have the same type rating as the 737 Classic so that 737 Classic pilots would not need costly recertification to fly the 737NG. In addition, the airlines wanted the 737NG to have the same basic design and same airframe as the 737 Classic to utilize the same mechanics, tooling, and spare parts as the 737 Classic.
In accordance with this direction, the 737NG has the same—or nearly the same—airframe, wing area, and control surfaces as the 737 Classic, and the size of the 737 Classic elevators (a control surface on the horizontal tail wings of an aircraft that control pitch) were not changed on the larger and heavier 737NG. However, the 737NG has more powerful engines and higher gross weights than the 737 Classic, and the 737NG therefore requires more aerodynamic authority then the existing 737 Classic elevators were capable of developing. Consequently, with reference FIG. 2, engineers developed dual-functioning elevator tabs 10a, 10b that are disposed or positioned adjacent to a trailing edge of each elevator 12a, 12b on each horizontal stabilizer 14a, 14b of the 737NG and that are displaceable relative to the respective elevator 12a, 12b. During normal flight operations, the elevator tabs 10a, 10b may function in a “balance” mode (illustrated in FIGS. 4B and 4D) as a portion of the elevators 12a, 12b. However, the elevator tabs 10a, 10b are automatically reversed to an “anti-balance” function (illustrated in FIGS. 4A and 4C) to displace relative to each corresponding elevator 12a, 12b in two specific flight conditions: (1) hydraulics engaged; and (2) flaps not retracted.
The “balance” function of the elevator tabs 12a, 12b relates to the redundant flight control functions of the 737NG. The 737NG has two primary hydraulic systems, but the 737NG is capable of operating with one or even both of those hydraulic systems failed. In the case of dual failure of the hydraulic systems, the pilot can still control the 737NG by physical strength combined with the help of aerodynamic and mechanical devices and couplings. In such a scenario, and as illustrated in FIGS. 4B and 4D, the “balance” function of the elevator tabs 10a, 10b displaces the elevator tabs 10a, 10b in opposition to displacement of the elevators 12a, 12b. That is, when the elevator 12a, 12b (i.e., a trailing edge 28a, 28b of each elevator) pivots upwardly (as illustrated in FIG. 4B), the corresponding elevator tab 10a, 10b (i.e., the trailing edge of the elevator tabs 10a, 10b) pivots downwardly, and when the elevator 12a, 12b pivots downwardly (as illustrated in FIG. 4D), the corresponding elevator tab 10a, 10b pivots upwardly. This opposition movement applies an assisting load to the elevator surface allowing the pilot to move the elevator 12a, 12b when operating without hydraulic power. If the “balance” function of the elevator tabs 10a, 10b was to fail when required, the 737NG could not be manually controlled by a pilot.
A second elevator tab engagement scenario involves a takeoff from a runway with a limited length when an engine fails just after the aircraft has achieved V1 speed (the speed reached during takeoff where it is just possible to stop the aircraft with the remaining distance of runway). If V1 speed is exceeded, the aircraft is required to complete the takeoff or it will overrun the remaining runway if the takeoff is aborted. In such a takeoff, the elevator tabs 10a, 10b perform an “anti-balance” function in which the elevator tabs 10a, 10b displace in concert with the elevators 12a, 12b. That is, when the elevator 12a, 12b (i.e., the trailing edge of the elevator 12a, 12b) pivots upwardly, the corresponding elevator tab 10a, 10b (i.e., the trailing edge of the elevator tabs 10a, 10b) pivots upwardly (as illustrated in FIG. 4A), and when the elevator 12a, 12b pivots downwardly, the corresponding elevator tab 10a, 10b pivots downwardly (as illustrated in FIG. 4C). This in-concert displacement movement generates a greater elevator surface hinge moment than the elevator of a 737 Classic, thereby allowing the elevators 12a, 12b and elevator tabs 10a, 10b to rotate the aircraft before reaching the end of that runway. As with the “balance” function, if the “anti-balance” function was to fail when required, the aircraft would not have sufficient control authority to be assured of maintaining safe, continued flight.
While the elevator tabs perform a critical function in the two scenarios described above, the operation of the elevator tabs is entirely controlled by computer, and the pilots have no ability to manually or specifically control their operation. In addition, because the elevator tabs are not a part native to the 737 Classic, no instrumentation is connected to or in communication with the elevator tabs to directly detect failures, and the pilot (and flight computer) has no indication that the elevator tabs are functioning (or can function) properly. Accordingly, there is a need for a method or system to indirectly detect proper operation of the elevator tabs to ensure that the “balance” or “anti-balance” functions are available in an emergency.